impatient rot at base of plant

For the past few years around this time many of my impatients begin to wilt and then completely disinegrate. At the base of the stem before it goes into the dirt it turns black and gets rotten. Eventually the whole plant goes. I have tried new soil every year, less miracle grow,less frequent waterings, not planting too close together etc. It happens to them in the ground and in every type of container. Any suggestions?